This invention relates generally to a high frequency switching power converter, and more particularly to a switching converter capable of concurrently inverting and rectifying. A preferred embodiment is described in which the switching power converter may provide the interface between a DC wind generator or solar panel array and a conventional AC power grid.
The switching power converter circuit takes the form of a high frequency switching inverter, implemented with bidirectional switches capable of blocking negative voltages to enable rectification of AC power as well as inversion of DC power. This is accomplished by providing a transformer whose primary winding is center-tapped and conventionally inverted by switching devices, high power field-effect transistor (FET) switching circuitry connected between the transformer's secondary and current storage means, and a digital switch controller that generates pulse width modulated (PWM) signals for synchronizing the conventional inverter and the high power switching circuitry. The circuit may be connected between a DC source and an AC load, between a DC load and an AC source, or between more general DC and AC elements The switching power converter may, therefore, be used as both a battery charger and an AC source.
Switching inverters are frequently used to power an AC appliance from a DC source, e.g. batteries or solar arrays. Inductive AC loads, e.g. motors, require relatively high starting currents usually provided by high power inverters. Typically, high current is required only during the start-up of the motor, thus much of the capacity of the conventional high power inverter is wasted once the motor reaches a steady state operating condition. Such high power inverters are generally linear rather than switching devices, rendering them relatively costly and inefficient. When the inverter's DC power source fails, the AC appliance ceases to operate.
The typical rectifier circuit enables the supply of DC power from an AC source. A rectifier can be used, for example, to recharge a battery or directly to drive a DC appliance from an AC line. The charging of a battery using a rectifier requires vigilance on the part of the operator, as batteries are susceptible of exploding when overcharged. Trickle charging of batteries, sometimes referred to as "topping," is a tedious process, and removes the DC power supply from operation for quite some time. Charging a battery generally requires the removal of the DC power supply from its operating environment.
The problems associated with using an inverter to convert battery power for use by an AC appliance, and using a separate rectifier to convert AC power for recharging a discharged battery are numerous. At the very best, this dual purpose may be served by conventional means only with manual intervention. For example, if it is determined that the battery has discharged, it is necessary to remove the battery from the circuit to which it supplies power and then to connect it to the rectifier for recharging. Integral converter means by which one may automatically convert DC power to AC power and vice versa have been theorized but are heretofore unrealized.
In the preferred embodiment of the instant invention, a switching converter circuit is provided that has a transformer with a center-tapped primary connectable to the positive terminal of a DC power supply. The end taps of the primary are controllably alternately connected to the negative (or common) side of the DC power supply, providing alternately polarized stepped-up voltage pulses on the transformer's secondary winding.
An array of bidirectional power FET switches is controlled synchronously with the primary's switches alternately to supply positive and negative PWM signals to the two input terminals of an inductor. The PWM signals are provided by a programmable ROM-based table of values. Thus, the PWM waveform may represent a sinusoid or any other desired symmetric shape.
The ROM contains a number of tables representing waveforms of various amplitudes. Analog sense circuitry provides feedback to decision logic that, in turn, selects a base address in ROM for a higher or lower amplitude waveform Thus ROM-based digital regulation is provided, dynamically responsive to a variable load. A no-load sensing circuit automatically selects a ROM base address of a table containing a reduced pulse width and duty cycle control signal for the power FET switch array.
High current protection also is provided, by a circuit that senses when the power through the power FET switches exceeds a predefined level. When it does, the digital pulse stream used to control the power FET switches is altered to produce a clamped sinusoidal output.
Each switch in the power FET switch array is powered independently from any other and from ground by a separate tap on the transformer's secondary coil. While blocking reverse voltages, each switch is capable, by virtue of an integral shunted diode, of conducting current in either direction. This novel feature allows the switches to operate bidirectionally, and allows the converter circuit, in turn, to act concurrently as both inverter and rectifier.
It can be seen that such a circuit provides for efficient, automatic inversion and rectification, as required in interfacing DC and AC power grids. When, for example, the DC power grid has more capacity than the AC power grid, phase-locked AC power may be supplied to the AC grid. Similarly, when the AC power grid has more capacity than the DC power grid, DC power may be supplied to the DC grid. When connected between a DC power supply and a reactive load, the dynamic clamping feature will protect the circuit's power switches while responding to the variable load. The switching nature of the converter makes it energy efficient and cool in operation. These and other objects and advantages of the present invention will be more clearly understood from a consideration of the drawings and the following description of the preferred embodiment.